Diffractive optical element for providing favorable multi-mode fiber launch and reflection management

ABSTRACT

A light transmission system includes a laser, an optical fiber, and a transfer lens. The transfer lens transfers light emitted by the laser into the optical fiber. The transfer lens includes a diffractive surface for receiving and collimating the light originating form the laser. The diffractive surface is defined by a surface function that includes a first phase function having angular symmetry and a second phase function having radial symmetry. The second phase function includes a cusp region with a discontinuous slope therein. The transfer lens provides reflection management so that light reflected from the end of the optical fiber is not focused at a location at which light is emitted by the laser and also favorable launch conditions so that light launched into the optical fiber avoids index anomalies along the axis of the optical fiber.

FIELD OF THE INVENTION

[0001] The present invention relates generally to optics, and moreparticularly, to a diffractive optical element that provides favorablemulti-mode fiber launch and reflection management.

BACKGROUND OF THE INVENTION

[0002] A vertical cavity surface emitting laser (VCSEL) emits light in abeam vertically from it surface. Light emitted from a VCSEL is typicallyfocused by a transfer lens into an optical fiber and used for thetransmission of data. Transmission technology such as Gigabit Ethernettechnology utilizes VCSELs and multimode fiber optic cabling.

[0003] The ever-increasing data rate across multimode fiber opticsystems requires more sophisticated coupling optics for the transmittermodule to satisfy the required bit-error rate.

[0004] There are two important considerations in the design of atransfer lens: 1) reflection management, and 2) creation of a favorablelaunch condition. The first design consideration of reflectionmanagement seeks to minimize the amount of light that is reflected backfrom the surface of the optical fiber (referred to as “back reflections”or feedback) and directed to the light source (e.g., the laser). Whenreflections are not managed properly, the back reflections can causestability problems for the laser source. Specifically, if these backreflections are not controlled or reduced, the laser can becomede-stabilized and may operate with a noisy output signal. For example,when too much power is coupled back into the laser from the reflectionfrom the end of the optical fiber, instabilities occur in the laser, andthe output power oscillates up and down, thereby causing extra anddamaging amounts of jitter as the received signal pulses. In otherwords, instability in the laser causes erroneous data signals.

[0005] Furthermore, the increased noise in the laser that is induced bythe coupling lens can lead to a power penalty in the optical budget ofthe data link as high as 2.5 dB. It is evident that the increased powerpenalty due to the back reflections represents a significant fraction ofthe total link power budget which, for a 2.5 Gbit/sect data rate, is onthe order of about 8 dB. This adverse effect of back reflections orfeedback becomes more pronounced and significant for higher data ratesystems. For example, the power budget for a 10 Gbit/sec link becomeseven more taxed than the 2.5 Gbit/sec link.

[0006] Second, it is important that the transfer lens design provide afavorable launch condition at the fiber interface in order to maximizethe bandwidth-distance product of the system. For example, for astandard 50 micron graded-index fiber, a 2.5 Gbit/sec link requires abandwidth-distance product of 500 MHz*km. Similarly, for a 10 Gbit/seclink, the fiber needs to support a product of 2.2 Ghz*km.

[0007] A favorable launch condition should increase bandwidth of thesystem and is robust to lateral offsets (i.e., misalignment between thelaser and the fiber). One approach to improve favorable launch conditionis to avoid launching the light along the very center of the fiber. Areason for avoiding the center of the fiber is that many fibers havedefects along the center of the fiber due to manufacturing limitations.Furthermore, tolerance for lateral offsets is desirable to compensatefor any misalignment between the laser and the fiber. Otherwise,misalignment in the system (e.g., misalignment between the optical fiberand transfer lens or the misalignment between the transfer lens and thelaser) may cause the light from the laser to miss the optical fiber.

[0008] Unfortunately, the prior art transfer lens designs haveshortcomings in either addressing back reflections or providingfavorable launch conditions. These shortcomings and disadvantages stemprimarily from constraints and difficulties in lens fabrication.

[0009] Diffractive Vortex Lens for Mode-Matching Graded Index Fiber

[0010] There have been some attempts to use diffractive elements ascoupling optics to launch light into graded index fiber. One such studyis reported by E. G. Johnson, J. Stack, C. Koehler, and T. Suleski inthe Diffractive Optics and Micro-Optics, Optical Society of America(OSA) Technical Digest, pp. 205-207, Washington, D.C., 2000, in anarticle entitled, “Diffractive Vortex Lens for Mode-Matching GradedIndex Fiber.” This publication describes an approach that utilizes adiffractive element to match the phase of the launched light intospecific modes of the graded index fiber.

[0011] While these prior art approaches provide tolerable results forideal point light sources (i.e., light that has a simple distributionand is perfectly coherent), these approaches do not adequately addressapplications that employ light sources with more complex lightdistributions (e.g., a multi-mode laser). In these specific real-worldapplications, the prior transfer lens suffer from more destabilizingfeedback due to poor management of back reflections, unfavorable launchconditions stemming from larger amounts of on-axis energy, or both.

[0012] Based on the foregoing, there remains a need for a transfer lensthat simultaneously reduces back reflection and provides favorablelaunch conditions and that overcomes the disadvantages set forthpreviously.

SUMMARY OF THE INVENTION

[0013] According to one embodiment of the present invention, a transferlens for transferring light emitted by a light source (e.g., a laser)into a light conducting medium (e.g., an optical fiber) is provided. Thetransfer lens includes a diffractive surface for receiving andcollimating the light originating from the light source. The diffractivesurface is defined by a surface function that includes a first phasefunction having angular symmetry and a second phase function havingradial symmetry. The second phase function includes a cusp region with adiscontinuous slope therein. The transfer lens provides reflectionmanagement so that light reflected from the end of the optical fiber isnot focused at a location at which light is emitted by the light source.Furthermore, the transfer lens also provides favorable launch conditionsso that light launched into the optical fiber avoids index anomalies onthe axis of the optical fiber and at the core-cladding interface.

[0014] A further advantage of the transfer lens design of the presentinvention is that the diffractive surface of the transfer lens providesreflection management and favorable launch conditions, which isparticularly advantageous for multi-mode fiber optic systems.

[0015] In accordance with one embodiment of the present invention, alight transmission system that includes a laser, an optical fiber, and atransfer lens is provided. The transfer lens transfers light emitted bythe laser into the optical fiber. The transfer lens includes adiffractive surface for receiving and collimating the light originatingform the laser. The diffractive surface is defined by surface functionthat includes a first phase function having angular symmetry and asecond phase function having radial symmetry. The second phase functionincludes a cusp region with a discontinuous slope therein. The transferlens provides reflection management so that light reflected from the endof the optical fiber is not focused at a location at which light isemitted by the laser and also favorable launch conditions so that lightlaunched into the optical fiber avoids index anomalies along the axis ofthe optical fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention is illustrated by way of example, and notby way of limitation, in the figures of the accompanying drawings and inwhich like reference numerals refer to similar elements.

[0017]FIG. 1 is a simplified block diagram of an exemplary lighttransmission system in which the transfer lens can be utilized inaccordance with a preferred embodiment of the present invention.

[0018]FIG. 2A illustrates an exemplary first phase function havingangular symmetry. FIG. 2B illustrates an exemplary second phase functionhaving radial symmetry. FIG. 2C illustrates a function that combines thefirst phase function of FIG. 2A and the second phase function of FIG. 2Bin accordance with a preferred embodiment of the present invention.

[0019]FIG. 3 illustrates a perspective view of a preferred diffractivesurface of the transfer lens that has stair case features.

[0020]FIG. 4 illustrate cross-sections of exemplary phase functionshaving radial symmetry and a cusp region that can be incorporated intothe diffractive surface of the transfer lens in accordance withembodiments of the present invention.

[0021]FIG. 5 is a spot diagram for the transfer lens of the presentinvention for illustrating favorable launch conditioning.

[0022]FIG. 6 is a spot diagram for the transfer lens of the presentinvention for illustrating reflection management.

[0023]FIG. 7 is a spot diagram for a prior art transfer lens forillustrating poorer launch conditioning.

[0024]FIG. 8 is a spot diagram for a prior art transfer lens forillustrating poor reflection management.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] A diffractive optical element optimized for multi-mode fiberlaunch and feedback control is described. In the following description,for the purposes of explanation, numerous specific details are set forthin order to provide a thorough understanding of the present invention.It will be apparent, however, to one skilled in the art that the presentinvention may be practiced without these specific details. In otherinstances, well-known structures and devices are shown in block diagramform in order to avoid unnecessarily obscuring the present invention.

[0026] Light Transmission System 100

[0027]FIG. 1 is a simplified block diagram of an exemplary lighttransmission system 100 in which the transfer lens 140 can be utilizedin accordance with a preferred embodiment of the present invention. Thesystem 100 includes a light source 110 (e.g., a laser) for emittinglight, a light conducting medium 120 (e.g., a fiber optic cable), and atransfer lens 140 of the present invention for transferring lightemitted by the light source 110 to the light conducting medium 120.

[0028] The light source 110 can be a semiconductor laser, such as avertical cavity surface emitting laser (VCSEL), whose construction andoperation are known to those of ordinary skill in the art. The lightconducting medium 120 can, for example, be a 50 micron multimode fiberor a 62.5 micron multi-mode fiber, which are well-known types of fiberoptics cables. The transfer lens 140 of the present invention furtherprovides favorable launch conditions and reflection management (alsoreferred to herein as feedback management).

[0029] In the preferred embodiment, the transfer lens 140 includes adiffractive surface 150 for receiving and collimating the lightoriginating form the light source and also for providing feedbackmanagement and favorable launch conditions. The transfer lens 140includes an optical surface 160 (e.g., a refractive surface or adiffractive surface) for providing magnification of the light andfocusing the light onto the light conducting medium 120. The transferlens 140 of the present invention is described in greater detailhereinafter with reference to FIGS. 2-6.

[0030] One aspect of the transfer lens of the preferred embodiment isplacing the diffractive surface 150 inside the packaging so that thediffractive surface 150 is less susceptible to dust and damage.

[0031] Diffractive Surface 150

[0032] The diffractive surface 150 of the transfer lens shown in FIG. 1is defined by a surface function. In accordance with a preferredembodiment of the present invention, the surface function for thediffractive surface 150 includes 1) a first phase function where thephase is dependent only on the polar angle coordinate of the aperture(herein referred to as a first phase function having “angularsymmetry”), and 2) a second phase function where the phase is dependentonly on the polar radial coordinate of the aperture (herein referred toas a phase function having “radial symmetry”) and further having a cuspregion. Preferably, the second phase function is symmetric with respectto an imaginary line drawn through the center of the aperture, and thecusp region has a discontinuous slope contained therein.

[0033] Examples of a first phase function having angular symmetry and asecond phase function having radial symmetry are now described. FIG. 2Aillustrates an exemplary first phase function having angular symmetry(e.g., a spiral phase function where m_(S)=+3). m_(S) is a real number(from −INF to +INF) that describes how fast the phase changes as onetraverses a circle about the center of the aperture.

[0034]FIG. 2B illustrates an exemplary second phase function havingradial symmetry (e.g., a cone phase function where m_(C)=−2). m_(C) is areal number (from −INF to +INF) that describes how fast the phasechanges as one traverses a radial line from the center of the aperture.The slope of the cone controls the change in phase values from 0 at thecenter of the aperture to 2*pi*m at the edge. Further examples of secondphase function having radial symmetry and a cusp region are describedwith reference to FIG. 4.

[0035]FIG. 2C illustrates a function that combines the first phasefunction of FIG. 2A and the second phase function of FIG. 2B inaccordance with a preferred embodiment of the present invention. In thisembodiment, the particular “m” values are selected as follows: m=+3 forphase function having angular symmetry, and m=−2 for the phase functionhaving radial symmetry. The “m” values can be determined by balancingthe coupling efficiency, misalignment tolerances and the amount offeedback. It is noted that these “m” values may be adjusted to suit therequirements of a particular optical application.

[0036] It is noted that the surface function for the surface 150 caninclude other phase terms (e.g., a third phase function, a fourth phasefunction, etc.) to suit the requirements of a particular application.These additional phase functions or phase terms can include, forexample, lens functions, aberration control functions, prism functions,and grating functions.

[0037] These terms are known to those of ordinary skill in the art andwill not be described further herein.

[0038] In an alternative embodiment, the surface 150 can be acollimating surface for receiving and collimating the light originatingand the diffractive aspect of the present invention can be incorporatedin surface 160. In this embodiment, surface 160 is a diffractive surfacefor providing feedback management and favorable launch conditions forthe transfer lens.

[0039]FIG. 3 illustrates a perspective view of a preferred diffractivesurface of the transfer lens that has stair case features. It is notedthat the stair-like or step-like feature of the diffractive surface isespecially suited for manufacture by standard semiconductor processes.For example, well-known lithography involving masks and etch processingsteps can be employed to realize the diffractive surface of transferlens of the present invention. Alternatively, the diffractive surfacecan include continuous or smooth surface transitions. It is noted thatthe continuous or smooth surface transitions may lead to increasedperformance by the transfer lens of the present invention. However, thisalternative embodiment may require more complex processing steps thatinclude turning or milling the diffractive surface to achieve acontinuous slope.

[0040]FIG. 4 illustrates sectional views of exemplary phase functionshaving radial symmetry and a cusp region that can be incorporated intothe surface function of the diffractive surface of the transfer lens inaccordance with embodiments of the present invention. A first phasefunction 410 has a cross section that features a generally concaveprofile. A second phase function 420 features a generally triangularcross-section. A first phase function 430 has a cross section thatfeatures a generally convex profile. It is noted that each of thesephase functions can be inverted, and the inverted phase function can beutilized to achieve a similar result. For example, a fourth phasefunction 440 is an inverted version of the first phase function 410.

[0041] Preferred Embodiment

[0042] Preferably, the design of the diffractive element of the presentinvention includes at least two phase functions (i.e., the first phasefunction having angular symmetry combined with the second phase functionhaving radial symmetry) described above.

[0043] Describe the phase, φ, of all points within the lens aperturewith polar coordinates: ρ,θ, where ρ is the distance of the point fromthe center of the aperture, and θ is the angle of the point from thex-axis.

[0044] In the preferred embodiment, the surface function of thediffractive element of the present invention includes at least a firstphase function having angular symmetry (e.g., a spiral phase function)combined with a second phase function having radial symmetry (e.g., acone phase function). For example, the surface function can be expressedas follows:

φ=m _(S)*θ+2πm _(C)*ρ.

[0045] The spiral phase function can be expressed as follows:

φ=m_(S)*θ

[0046] where ‘m_(S)’ is a real number, −INF to +INF, that describes howfast the phase changes as one traverses a circle about the center of theaperture.

[0047] The cone phase function can be expressed as follows:

φ=2πm_(C)*ρ

[0048] where ‘m_(C)’ is a real number, −INF to +INF, that describes howfast the phase changes as one traverses a radial line from the center ofthe aperture. ρ is a normalized radial coordinate so that ρ is equal to1 at the edge of the aperture, and ρ is equal to zero at the center ofthe aperture.

[0049] As noted previously, other phase functions or phase terms may beadded to the above surface function to further describe the opticalproperties of the diffractive element.

[0050] FIGS. 5-8 shows simulated light distributions at the fiber andfeedback planes for the transfer lens of the present invention and theprior art transfer lens design for a multimode laser source (e.g., amultimode VCSEL source).

[0051]FIG. 5 is a spot diagram for the transfer lens of the presentinvention for illustrating favorable launch conditioning. It is notedthat the transfer lens of the present invention provides very efficientcoupling and also avoids the center of the fiber. Specifically, thediffractive surface 150 provides favorable launch conditions so thatlight launched into the optical fiber avoids the index anomalies on theaxis of the optical fiber and at the core-cladding interface. FIG. 6 isa spot diagram for the transfer lens of the present invention forillustrating reflection management. It is noted that the transfer lensof the present invention provides very good feedback management bydirecting reflections away from the light source.

[0052]FIG. 7 is a spot diagram for a prior art transfer lens thatexhibits increased energy launched along the axis of the fiber. Incontrast to FIG. 5, the prior art transfer lens provides poorer launchconditioning since there is more energy launched into the center of thefiber optic cable. As described previously, it is disadvantageous tolaunch energy into the center since the manufacturing defects thereinmay adversely affect or decrease the bandwidth of the system.

[0053]FIG. 8 is a spot diagram for a prior art transfer lens forillustrating poor reflection management. In contrast to FIG. 6, theprior art transfer lens provides poor reflection management since thereis more light reflected back into the light source. Consequently, thelight source may suffer from stability problems due to poorly managedfeedback.

[0054] In the foregoing specification, the invention has been describedwith reference to specific embodiments thereof. It will, however, beevident that various modifications and changes may be made theretowithout departing from the broader scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

[0055] For example, the diffractive element of the present invention hasbeen described in the context of launching light from a multi-mode laserinto a fiber optic medium. However, it is to be appreciated that thediffractive element of the present invention can be applied to manydifferent applications in a wide number of different areas. Thediffractive element of the present invention is beneficial to anyapplication where light needs to be coupled from a light source with acomplex light distribution to another light conducting medium. Forexample, the diffractive element of the present invention can beutilized to transfer light between two multi-mode fibers.

What is claimed is:
 1. A diffractive transfer lens for coupling a lightsource to a light conducting medium comprising: a diffractive surfacethat is defined by a surface function that includes a first phasefunction having angular symmetry, and a second phase function havingradial symmetry and a cusp region; wherein the cusp region has adiscontinuous slope therein.
 2. The transfer lens of claim 1 wherein thefirst phase function is a spiral phase function; and wherein the secondphase function is a cone phase function.
 3. The transfer lens of claim 2wherein the spiral phase function can be expressed as follows:φ=m_(S)*θwhere ‘m_(S)’ is a real number that describes how fast thephase changes as one traverses a circle about the center of theaperture; and the cone phase function can be expressed as follows:φ=2πm_(C)*ρ where ‘m_(C)’ is a real number that describes how fast thephase changes as one traverses a radial line from the center of theaperture.
 4. The transfer lens of claim 3 wherein ms is equal to =3 andm_(C) is equal to −2.
 5. The transfer lens of claim 1 wherein thetransfer lens provides reflection management so that light reflectedfrom the end of the optical fiber is not directed to a location at whichlight is emitted by the laser.
 6. The transfer lens of claim 1 whereinthe transfer lens provides favorable launch conditions so that lightlaunched into the optical fiber avoids index anomalies along the axis ofthe optical fiber.
 7. The transfer lens of claim 1 further comprising:an optical surface for focusing the light onto the optical fiber; andwherein the diffractive surface receives and collimates the lightoriginating from a light source.
 8. The transfer lens of claim 1 furthercomprising: a packaging for housing the light source; wherein thediffractive surface is disposed in the housing.
 9. An optical module forcoupling to an optical fiber comprising: a laser for emitting light; atransfer lens for transferring light emitted by the laser into theoptical fiber; wherein the transfer lens includes a diffractive surfacethat is defined by a surface function; wherein the surface functionincludes a first phase function combined with a second phase functionfor providing favorable launch conditions and reflection management. 10.The optical module of claim 9 wherein the first phase function hasangular symmetry; and wherein the second phase function has radialsymmetry and a cusp region with a discontinuous slope.
 11. The opticalmodule of claim 9 wherein the transfer lens provides reflectionmanagement so that light reflected from the end of the optical fiber isnot directed to a location at which light is emitted by the laser. 12.The optical module of claim 9 wherein the transfer lens providesfavorable launch conditions so that light launched into the opticalfiber avoids index anomalies along the axis of the optical fiber. 13.The optical module of claim 9 wherein the optical module is one of anoptical receiver, an optical transmitter, and an optical transceiver.14. The optical module of claim 9 wherein the first phase function is aspiral phase function; and wherein the second phase function is a conephase function.
 15. The optical module of claim 10 wherein the spiralphase function can be expressed as follows: φ=m_(S)*θwhere ‘m_(S)’ is areal number that describes how fast the phase changes as one traverses acircle about the center of the aperture; and the cone phase function canbe expressed as follows: φ=2πm_(C)*ρ where ‘m_(C)’ is a real number thatdescribes how fast the phase changes as one traverses a radial line fromthe center of the aperture.
 16. The optical module of claim 15 whereinms is equal to =3 and m_(C) is equal to −2.
 17. The optical module ofclaim 9 further comprising: an optical surface for focusing the lightonto the optical fiber; and wherein the diffractive surface receives andcollimates the light originating form the laser.
 18. The transfer lensof claim 9 further comprising: a packaging for housing the light source;wherein the diffractive surface is disposed in the housing.
 19. A methodfor manufacturing a diffractive surface for use in a transfer lenscomprising: defining a first phase function having angular symmetry;defining a second phase function having radial symmetry and a cuspregion; wherein the cusp region has a discontinuous slope therein;defining a surface function that includes the first phase function andthe second function; and employing semiconductor processing techniquesto manufacture a diffractive surface for use in the transfer lens inaccordance with the surface function.
 20. The method of claim 19 furthercomprising: adding a third phase function to the surface function;wherein the third phase function includes one of a lens phase function,an aberration control phase function, a prism phase function, and agrating phase function.